Oracle 18c新特性-PDB快照轮播(Snapshot Carousel)

oracle 18c推出的一个新功能就是pdb快照轮播，最多可以创建8个pdb的快照，而这8个构成的一组快照就被称作快照轮播。当达到第8个快照限制时，新的快照就会覆盖最旧的快照，就像redo log一样。pdb的快照主要作用是当出现了问题可以闪回到快照的时间点，另一个功能是基于快照来查询历史数据。

sql> select r.con_id, p.pdb_name, property_name,
    property_value as value, description
    from  cdb_properties r, cdb_pdbs p
    where r.con_id = p.con_id
    and  property_name like 'max_pdb%'
    and  description like 'maximum%'
    order by property_name;

  con_id pdb_name       property_name    value        description
---------- -------------------- -------------------- -------------------- ----------------------------------------------------------------------
     3 orclpdb1       max_pdb_snapshots            
maximum number of snapshots for a given pdb

默认情况下是手动创建快照，我们可以把它改成自动创建。

sql> select snapshot_mode,snapshot_interval/60 from dba_pdbs;
snapsh snapshot_interval/60
------ --------------------
manual
sql> alter pluggable database snapshot mode every 4 hours;
pluggable database altered.
sql> select snapshot_mode,snapshot_interval/60 from dba_pdbs;
snapsh snapshot_interval/60
------ --------------------
auto  
4

要返回手动模式，输入mode manual即可。

sql> alter pluggable database snapshot mode manual;
pluggable database altered.
sql> alter pluggable database snapshot;
pluggable database altered.

手动创建快照，可以自己指定名字，也可以使用系统自动生成的名字。通过查看dba_pdb_snapshots视图，可以找到快照存放的路径及创建快照的scn号。

sql> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, full_snapshot_path from dba_pdb_snapshots order by snap_scn;
  con_id con_name       snapshot_name        snap_scn full_snapshot_path
---------- -------------------- ------------------------- ---------- ---------------------------------------------------------------------------
     3 orclpdb1       snap_2953839490_989014667  2821702 /opt/oracle/oradata/orclcdb/orclpdb1/snap_2953839490_2821702.pdb
     3 orclpdb1       pdb_snap           2823303 /opt/oracle/oradata/orclcdb/orclpdb1/snap_2953839490_2823303.pdb
     3 orclpdb1       snap_2953839490_989015970  2823679 /opt/oracle/oradata/orclcdb/orclpdb1/snap_2953839490_2823679.pdb
[oracle@oracle-18c-vagrant orclpdb1]$ ls -lsh
total 1.3g
4.0k drwxr-x---. 8 oracle oinstall 4.0k oct 8 22:39 orclcdb
171m -rw-r--r--. 1 oracle oinstall 171m oct 8 22:18 snap_2953839490_2821702.pdb
171m -rw-r--r--. 1 oracle oinstall 171m oct 8 22:38 snap_2953839490_2823303.pdb
171m -rw-r--r--. 1 oracle oinstall 171m oct 8 22:40 snap_2953839490_2823679.pdb
371m -rw-r-----. 1 oracle oinstall 371m oct 8 22:39 sysaux01.dbf
271m -rw-r-----. 1 oracle oinstall 271m oct 8 22:39 system01.dbf
 56k -rw-r-----. 1 oracle oinstall 63m oct 1 23:21 temp01.dbf
101m -rw-r-----. 1 oracle oinstall 101m oct 8 22:39 undotbs01.dbf
5.1m -rw-r-----. 1 oracle oinstall 5.1m oct 8 22:39 users01.dbf

如果要删除快照，则有两种方法，第一种就是使用alter pluggable database删除，第二种是修改max_pdb_snapshots参数，当max_pdb_snapshots参数设置成0时，将删除全部快照。

sql> alter pluggable database drop snapshot snap_2953839490_989015970;
pluggable database altered.
sql> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, snapshot_time, full_snapshot_path from dba_pdb_snapshots order by snap_scn;
  con_id con_name       snapshot_name        snap_scn snapshot_time full_snapshot_path
---------- -------------------- ------------------------- ---------- ------------- ---------------------------------------------------------------------------
     3 orclpdb1       snap_2953839490_989014667  2821702  1539008272 /opt/oracle/oradata/orclcdb/orclpdb1/snap_2953839490_2821702.pdb
     3 orclpdb1       pdb_snap           2823303  1539009503 /opt/oracle/oradata/orclcdb/orclpdb1/snap_2953839490_2823303.pdb
sql> alter pluggable database set max_pdb_snapshots=0;
pluggable database altered.
sql> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, snapshot_time, full_snapshot_path from dba_pdb_snapshots order by snap_scn;
no rows selected

演示了上述功能之后，还有一个功能，就是我们可以根据快照创建pdb。

sql> alter pluggable database set max_pdb_snapshots=8;
pluggable database altered
sql> alter pluggable database snapshot pdb_snap;
pluggable database altered.
sql> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, snapshot_time, full_snapshot_path from dba_pdb_snapshots order by snap_scn;
  con_id con_name       snapshot_name        snap_scn snapshot_time full_snapshot_path
---------- -------------------- ------------------------- ---------- ------------- ---------------------------------------------------------------------------
     3 orclpdb1       pdb_snap           2825283  1539010328 /opt/oracle/oradata/orclcdb/orclpdb1/snap_2953839490_2825283.pdb
sql> !
[oracle@oracle-18c-vagrant ~]$ mkdir -p /opt/oracle/oradata/orclcdb/orclpdb2
sql> create pluggable database orclpdb2 from orclpdb1 using snapshot pdb_snap create_file_dest='/opt/oracle/oradata/orclcdb/orclpdb2'; 
create pluggable database orclpdb2 from orclpdb1 using snapshot pdb_snap create_file_dest='/opt/oracle/oradata/orclcdb/orclpdb2'
*
error at line 1:
ora-65040: operation not allowed from within a pluggable database    
sql> alter session set container=cdb$root;
session altered.
sql> create pluggable database orclpdb2 from orclpdb1 using snapshot pdb_snap create_file_dest='/opt/oracle/oradata/orclcdb/orclpdb2'; 
pluggable database created.
sql> show pdbs;
  con_id con_name            open mode restricted
---------- ------------------------------ ---------- ----------
     2 pdb$seed            read only no
     3 orclpdb1            read write no
     5 orclpdb2            mounted
sql> alter pluggable database orclpdb2 open; 
pluggable database altered.

这样我们就根据快照创建了一个新的pdb，假设当前pdb出现了数据覆盖，我们就可以使用这个历史快照的pdb来恢复数据。

当然还有一个很好用的功能就是我们在创建pdb的过程中把他设置成15分钟产生一个快照。当然配置这个功能有一个先决条件：cdb必须处于本地undo模式.

sql> create pluggable database orclpdb3 from orclpdb1 file_name_convert=('orclpdb1','orclpdb3') snapshot mode every 5 minutes;
pluggable database created.

可以看到每隔5分钟自动创建一个快照

[oracle@oracle-18c-vagrant orclpdb3]$ ls -lrt snap*
-rw-r--r--. 1 oracle oinstall 179134736 oct 8 23:13 snap_2091710291_2830531.pdb
-rw-r--r--. 1 oracle oinstall 179167158 oct 8 23:18 snap_2091710291_2832545.pdb
-rw-r--r--. 1 oracle oinstall 179285979 oct 8 23:23 snap_2091710291_2833104.pdb
-rw-r--r--. 1 oracle oinstall 179284365 oct 8 23:28 snap_2091710291_2833687.pdb
-rw-r--r--. 1 oracle oinstall 179302205 oct 8 23:33 snap_2091710291_2834965.pdb
-rw-r--r--. 1 oracle oinstall 179315484 oct 8 23:38 snap_2091710291_2836695.pdb
-rw-r--r--. 1 oracle oinstall 115724288 oct 8 23:43 snap_2091710291_2837222.pdb

我们可以做个实验测试一下。在生成快照之前创建一个表插入点数据，这里上一次快照是23:43分。

sql> alter session set container=orclpdb3;
session altered.
sql> create table a1 as select * from dba_objects;
table created.
sql> select count(1) from a1;
 count(1)
----------
   72897
[oracle@oracle-18c-vagrant orclpdb3]$ ls -lrt snap*
-rw-r--r--. 1 oracle oinstall 179134736 oct 8 23:13 snap_2091710291_2830531.pdb
-rw-r--r--. 1 oracle oinstall 179167158 oct 8 23:18 snap_2091710291_2832545.pdb
-rw-r--r--. 1 oracle oinstall 179285979 oct 8 23:23 snap_2091710291_2833104.pdb
-rw-r--r--. 1 oracle oinstall 179284365 oct 8 23:28 snap_2091710291_2833687.pdb
-rw-r--r--. 1 oracle oinstall 179302205 oct 8 23:33 snap_2091710291_2834965.pdb
-rw-r--r--. 1 oracle oinstall 179315484 oct 8 23:38 snap_2091710291_2836695.pdb
-rw-r--r--. 1 oracle oinstall 179353508 oct 8 23:43 snap_2091710291_2837222.pdb
-rw-r--r--. 1 oracle oinstall  5767168 oct 8 23:48 snap_2091710291_2839217.pdb
sql> select con_id, con_name, snapshot_name, snapshot_scn as snap_scn, snapshot_time, full_snapshot_path from dba_pdb_snapshots order by snap_scn;
  con_id con_name       snapshot_name        snap_scn snapshot_time full_snapshot_path
---------- -------------------- ------------------------- ---------- ------------- ---------------------------------------------------------------------------
     7 orclpdb3       snap_2091710291_989017984  2830531  1539011592 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2830531.pdb
     7 orclpdb3       snap_2091710291_989018284  2832545  1539011887 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2832545.pdb
     7 orclpdb3       snap_2091710291_989018584  2833104  1539012188 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2833104.pdb
     7 orclpdb3       snap_2091710291_989018884  2833687  1539012489 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2833687.pdb
     7 orclpdb3       snap_2091710291_989019184  2834965  1539012787 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2834965.pdb
     7 orclpdb3       snap_2091710291_989019484  2836695  1539013087 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2836695.pdb
     7 orclpdb3       snap_2091710291_989019784  2837222  1539013386 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2837222.pdb
     7 orclpdb3       snap_2091710291_989020084  2839217  1539013686 /opt/oracle/oradata/orclcdb/orclpdb3/snap_2091710291_2839217.pdb

23点48分产生了新的快照，此时我们使用新的快照做一个克隆，登陆到orclpdb4数据库后，我们会发现是有数据的。

sql> alter session set container=cdb$root;
session altered.
sql> create pluggable database orclpdb4 from orclpdb3 using snapshot snap_2091710291_989020084 create_file_dest='/opt/oracle/oradata/orclcdb/orclpdb4';
pluggable database created
sql> alter pluggable database orclpdb4 open;
pluggable database altered.
sql> alter session set container=orclpdb4;
session altered.
sql> select count(1) from a1;
 count(1)
----------
   72897

这个功能对于测试人员非常有用，例如移动电信现在有bcv环境，专门用于经分抽数及测试，而这个bcv环境是底层同步的一个克隆。使用18c数据库，我们就可以直接创建快照，然后使用快照克隆一个新数据库形成bcv环境给经分抽数及测试使用了。

总结

以上所述是小编给大家介绍的oracle 18c新特性-pdb快照轮播（snapshot carousel），希望对大家有所帮助